Assuming you are in the United States and have a public library nearby, you might go look to see if they have a Scott Catalog. Look up your stamps in the catalog and you can see if there is any value.
Assuming there is some value, you should know that stamps sell for 25% to maybe 50% of the catalog value, so don't get all excited if they appear to be valuable.
If you plan to sell them, the easiest way is to list them on eBay
or find a stamp dealer. You will need to scan them front and back (or photograph) if you list on eBay